It is very important to improve the green tensile strength and the green modulus of rubber used in rubber industries so as to manufacture products having accurate measurements according to their designs or improve processability. Therefore, improvement in the green properties of rubber compositions has been attempted.
Japanese Patent Application Laid Open No. 16,061/1980 discloses a rubber composition which comprises 100 parts by weight of natural rubber or a rubber blend consisting of at least 30 parts by weight of natural rubber and at most 70 parts by weight of a synthetic diene rubber and 0.1 to 1.0 parts by weight of a dinitroso compound to achieve accurate measurements of rubber products by protecting from flow rubber compositions which are placed adjacent to each other for example, as in producing a tire. The rubber composition has improved green properties, while hysteresis loss and cut growth property after vulcanization has not been improved.
U.S. Pat. No. 3,562,303 shows a process which comprises the steps of mixing a rubbery high-cis polymer or copolymer of isoprene with from about 0.1 to about 0.3 times by weight the amount of sulfur, and from about 0.1 to about 0.5 times by weight the amount of accelerator, masticating the mixture at temperatures on the order of 275.degree.-350.degree. F., cooling to temperatures below 220.degree. F. and further compounding the mixture with additional quantities of sulfur sufficient to effect full vulcanization.
It has been proposed to provide epoxidized copolymers from 1,3-cyclodiene and acyclic conjugated diene monomers so as to improve green tensile strength, moduli, and oil resistance properties. Such epoxidized copolymers are described in U.S. Pat. No. 4,131,653.
Ionogene functional groups such as sodium acrylate, ammonium fumarate and calcium ethylpropenate are employed in the main chain of rubbery diene polymers or copolymers to achieve high green tensile strength, in Japanese Application Laid Open No. 155,652/1977.
Japanese Patent Publication Nos. 41,281/1983 and 41,282/1983 teach that tertiary amines are employed in the main chains of a butadiene-styrene copolymer or butadiene-acrylonitrile copolymer during the polymerization process and halogenated organic compound such as 4,4'-bis-bromomethyldiphenyl, 4,4'-bis-bromomethyldiphenylmethane, 4,4'-bis-bromomethyldiphenylether and 2,6-bis-bromomethylnaphthalene are added to polymerized lactices just after polymerization so as to improve green tensile strength.
U.S. Pat. No. 4,020,115 and U.S. Pat. No. 3,951,936 disclose rubbery homopolymers and copolymers exhibiting crystallinity on stretching uncompounded and uncured, a low vinyl content and a high trans-1,4 content so as to have high green tensile strength.
U.S. Pat. No. 3,868,344 suggests that improved green tensile strength can be obtained by controlling the molecular weights of the polymers.
Block polymers have been proposed to achieve high green strength. U.S. Pat. No. 4,152,370 proposes to provide a linear copolymer having the formula A-B/A'-C, wherein terminal block A is a block of poly(monovinylarene) and terminal block C is a block of poly(1,3-butadiene) having a low vinyl content. U.S. Pat. No. 4,168,286 proposes tetrablock polymers having the configuration B-A.sub.1 -C-A.sub.2 and C-A.sub.1 -B-A.sub.2, wherein B is a low vinyl poly 1,3-butadiene block, A.sub.1 and A.sub.2 which can be the same or different are polymerized blocks of a monovinylarene monomer and C is a polymerized block of a conjugated diene.
Methods of pre-cure of green rubber compositions before vulcanization using some irradiators such as an electron irradiator are described in U.S. Pat. No. 3,933,553; U.S. Pat. No. 3,933,566; U.S. Pat. No. 4,166,883; U.S. Pat. No. 4,221,253; and U.S. Patent Disclosure Document T951,005.
As mentioned above, there are many methods to improve the green properties of rubber compositions. However, the methods using irradiators need a large plant and equipment investment. A method employing ionogene functional groups or tertiary amines in the main chain of rubbery diene polymers or copolymers cannot be applied to natural rubber. Methods increasing the crystallinity of polymers or methods providing block copolymers deteriorate the physical properties of vulcanized rubber compositions.
Pneumatic tires generate heat internally during running. Intense heat results in heat separation of the belt or carcass portion of the tires. High hysteresis loss usually increases the rolling resistance of pneumatic tires. Recently, pneumatic tires which decrease fuel consumption of vehicles have been required from the standpoint of natural resource conservation and energy saving.
It is known to decrease the amount of carbon black or to change the type of carbon black in rubber compositions to obtain low hysteresis loss in rubber compositions. However, this method results in not only low green tensile strength but also deteriorated physical properties of vulcanized rubber compositions, especially low cut growth property. Therefore, delays in improving both green properties such as green tensile strength and green modulus and physical properties after vulcanization such as hysteresis loss and cut growth have occurred.
Rubber products which are used under severe conditions, such as tires, usually include a number of different components. The physical property requirement of each component are usually different. Accordingly, most of the components are made of different rubber compositions.
In rubber products having a plurality of components, rubber composition flow between the adjacent components in the vulcanization process sometimes prevents accurate arrangement of the rubber products and the reinforcing components according to the desired design. Especially, during tire vulcanization, an innerlinear rubber component receives a tensile force resulting from an increase in additional inner pressure caused by expansion of a bladder. Further, this component is simultaneously pressed against carcass components made of a ply coating rubber composition and ply cords and is pressed into the ply coating rubber composition between the ply cords. This phenomenon sometimes separates the ply coating rubber composition from the ply cords and decreases the effective thickness of the innerliner rubber component as an air barrier. In the extreme case, the ply cords are exposed on the innerliner rubber component, and this results in ply separation or tube puncture and a bad appearance of the inner surface of the tires.
Many methods are proposed to protect the innerliner rubber component from being pressed into a ply coating rubber composition between the ply cords during vulcanization.
U.S. Pat. No. 3,933,553; U.S. Pat. No. 3,933,566; U.S. Pat. No. 4,166,883; U.S. Pat. No. 4,221,253; and U.S. Patent Disclosure Document T951,005 described above teach a method where an innerliner rubber component after sheeting with a calender is pre-cured with an irradiator such as an electron irradiator before the tire building and vulcanization process.
The methods described above, which are to improve green properties such as green modulus and green tensile strength, have an effect of protecting an innerliner rubber component from rubber flow to a ply coating rubber composition during vulcanization.
Japanese Patent Publication No. 16,061/1980 discloses that a rubber composition comprising natural rubber, butadiene rubber and a dinitroso compound such as poly-p-dinitrosobenzene and bisnitroso-4-phenyl-1,4-piperadine, which is employed in an innerliner rubber composition or a ply coating rubber composition, affects the rubber flow of the innerliner rubber composition into the ply coating rubber composition during vulcanization.
Japanese Patent Publication No. 16,062/1980 teaches that a rubber composition, which comprises natural rubber or a blend rubber of natural rubber and a butadiene rubber and a dioxime compound such as p-benzoquinone-dioxime and p,p'-dibenzoquinone-dioxime, is employed in one of an innerliner rubber composition or a ply coating rubber composition and a rubber composition, which comprises natural rubber or a blend rubber of natural rubber and a butadiene rubber and an oxidizing agent such as chloranil, lead dioxide and peroxide compounds, is employed in the other of an innerlinear rubber composition or a ply coating rubber composition. In the induction period of vulcanization, the dioxime compound and the oxidizing agent migrate to the boundary between the innerliner rubber composition and the ply coating rubber composition, react with each other and form a dinitroso compound. This dinitroso compound acts as a vulcanizer and fixes the boundary.